1. Field of the Invention
This invention relates to a process for preparing a polyamide foam which may be carried out in a continuous manner.
2. Description of the Prior Art
Two processes are known to produce polyamide foams. One process consists in foaming an already formed polyamide polymer by adding a foaming agent. The other consists in simultaneous foaming and polymerization by lactam ring opening polymerization of a starting mixture containing a foaming agent.
The latter process is potentially superior because the latter process consists in performing a ring opening polymerization of a lactam and foaming simultaneously. However, it has not been used commercially because it has been a high temperature process requiring large equipment for controlling heat. Moreover, the additives such as foaming agents, foam stabilizers or pigments which can be employed are limited because they must be stable at high temperatures.
The only known process for continuous production of which the present coinventors are aware is the one that Bayer AG (West Germany) discloses in Kunstostoff Bd 59, 1969, Heft, Pages 13-20; Belgian Pat. No. 707,969, Dutch Pat. No. 6,709,848 and U.S. Pat. No. 3,574,146. It is not utilized commercially.
In one description of the Bayer process, a formate such as sodium formate is used as a foaming agent and catalyst. This method involves a one-component system in which all of the ingredients are initially mixed in a container at a relatively low temperature, e.g. 80.degree. - 100.degree. C. In order for reaction to take place, it is necessary to rapidly increase the temperature to about 200.degree. C. This requires the use of a heat exchanger of very high capacity which markedly increases the engineering and operating costs of the process. Additionally, since the temperature in the container is necessarily high, it is not possible to use economical foaming agents such as volatile liquids or compounds which decompose to generate a gas, since these materials would vaporize or decompose prematurely. As a result, it is not possible to modify the density and other properties of the foam.
In another application of the Bayer process, as described, the reactants are initially separated into two compositions. One composition contains the selected lactam together with boron hydride as a catalyst. The second composition contains additional lactam plus an isocyanate promoter. The compositions are kept at a temperature of about 100.degree. - 140.degree. C., and the temperature is thereafter increased to above 160.degree. C. in the connecting pipes and the mixing vessels. Polymerization is completed in a separate container after discharge of the reaction mixture from the mixing vessel.
There are expensive engineering problems with this system because of the necessity for the stepwise elevation in temperature. Additionally, the boron hydride which is used as both catalyst and foaming agent is highly labile, and this introduces stability problems. With this process, as with the first process, there are limitations on the variability of the density and other properties of the foams which can be produced because volatile and decomposable foaming agents cannot be employed.
The deficiencies of the available processes for the production of polyamide foams has forced the art to actively seek other and more facile methods of production.